A common method for testing semiconductor integrated circuit chips is to contact a conductor on the chip with a conductive probe so that the voltage on the conductor can be measured. The probe itself, however, inevitably affects the circuit since it constitutes an electrical load on it. As microelectronic circuit densities increase, the deleterious effects of electrical loading also increase; also, it becomes more difficult to make a reliable contact to the conductor with the probe.
The Bloom et al., U.S. Pat. No. 4,681,449, granted Jul. 21, 1987, and the paper, "One Hundred GHz On-Wafer S-Parameter Measurements by Electrooptic Sampling," by R. Majidi-Ahy et al., 1989 IEEE MTT-S Digest, pp. 229-301, are examples of the literature describing the use of optical probing for making integrated circuit tests. Optical probing relies on the proximity to the conductor under test of a member which displays the Pockel's effect; electric fields extending into such a material are capable of modulating the polarization of a laser beam propagating through it. By directing a polarized laser beam through the Pockel's effect member and analyzing the polarization modulations of the exiting laser beam, one can characterize the voltage on the conductor and thereby test, or diagnose, the operation of the semiconductor chip. This technique is sometimes described as "non-invasive" since it avoids both loading and the possibility of damage due to physical contact; it is particularly applicable to group III-V semiconductor chips such as gallium arsenide, which inherently display the Pockel's effect. The laser probe beam is typically directed through the gallium arsenide and reflected back upon itself by a metal conductor, although, alternatively, the laser beam could be directed through the Pockel's effect material without being reflected.
Because the Pockel's effect medium modulates the polarization of the laser beam, the beam must initially be polarized. After it has been modulated, the polarization modulations of the beam must typically be converted to intensity modulations, which can then be converted by a photodetector to electrical signal modulations. The polarized light cannot be transmitted by an optical fiber because the fiber affects its polarization in ways that vary with time, temperature and stress due to bending; consequently, the laser beam is transmitted to the electronic device through free space. It would be desirable to reduce the complexity of laser beam probe apparatus, and considerable work to that end has been done and is being done in the industry; it would also be desirable to use an optical fiber for laser beam transmission.